The effects of inhibitors of prostaglandin (PG) synthesis (indomethacin, aspirin, naproxen) on parturition in the rat will be studied. The research will be proceed along the following lines: 1. Correlation of changes in the motility patterns of the rat uterus at various times during gestation with the amount of PGs released in vivo and in vitro. 2. Correlation of the changes in motility with alterations on the cellular electrical activity of the longitudinal and circular muscle of the rat at various times during gestation, comparing normal rats with those treated with PG synthesis inhibitors. Electrical activity will be recorded with microelectrode and sucrose gap techniques. Prostaglandins will be measured by radioimmunoassay.